1. Technical Field
The present disclosure relates to a device for protecting an integrated circuit against overvoltages, and more specifically against electrostatic discharges.
2. Discussion of the Related Art
FIG. 1 is an electric diagram of an example of a device for protecting an integrated circuit against overvoltages. This device comprises a zener diode 101 having its anode connected to a low power supply terminal or rail GND of the circuit to be protected (not shown), and having its cathode connected to a high power supply terminal or rail VCC of the circuit to be protected. The device of FIG. 1 further comprises a diode 103 forward-connected between terminal GND and an input-output pad IO1 of the circuit to be protected, a diode 104 forward-connected between pad IO1 and terminal VCC, a diode 105 forward-connected between terminal GND and an input-output pad IO2 of the circuit to be protected, and a diode 106 forward-connected between pad IO2 and terminal VCC.
The device of FIG. 1 enables removing overvoltages that may occur between any two of pads of rails IO1, IO2, VCC, and GND of the circuit to be protected. For positive overvoltages between IO1 and GND, IO1 and IO2, VCC and GND, IO2 and GND, IO2 and IO1, VCC and IO1, and VCC and IO2, the threshold for triggering the protection is equal to the avalanche voltage of zener diode 101 plus the forward voltage drop of one of diodes 103 to 106 (generally 0.6 V) or of two of diodes 103 to 106 in series (generally 1.2 V). For the other overvoltages, the triggering threshold is equal to the forward voltage drop of one of diodes 101 and 103 to 106 (generally 0.6 V).
Diodes 101 and 103 to 106 of the protection device of FIG. 1 are generally integrated on the same semiconductor chip which may be either an independent chip intended to be connected to terminals IO1, IO2, VCC, and GND of the circuit to be protected, or the actual chip containing the circuits to be protected.
FIG. 2 is an electric diagram of another example of a device for protecting an integrated circuit against overvoltages. The device of FIG. 2 comprises the same elements as the device of FIG. 1, and further comprises, between terminals VCC and GND of the circuit to be protected, that is, in parallel with zener diode 101, a capacitor 201.
For fast voltage variations (high frequency) between terminals VCC and GND, capacitor 201 behaves substantially as a closed circuit (short-circuit). Thus, in case of an abrupt overvoltage, for example, in case of an electrostatic discharge, capacitor 201 enables to start removing the overvoltage in the time interval preceding the turning-on of zener diode 101, that is, between the time when the overvoltage exceed the protection triggering threshold and the time when the zener diode becomes fully conductive by avalanche effect.
The device of FIG. 2 thus provides a better protection than the device of FIG. 1 and is generally preferred to the device of FIG. 1 to protect the most critical circuits.
However, a disadvantage of the device of FIG. 2 is that, in practice, unlike diodes 101 and 103 to 106 which are integrated on the same semiconductor chip, capacitor 201 is a discrete component. This forces to provide space and additional tracks on the printed circuit board where the circuit to be protected and the protection device are assembled, and additional solder operations on assembly.